1. Field of the Invention
The present invention relates to a data drive circuit for a current writing type AMOEL display panel.
2. Background of the Related Art
In general, there are two kinds of AMOEL (Active Matrix Organic ElectroLuminescent) pixel structures; a voltage writing type pixel structure and a current writing type pixel structure. The AMOEL display panel of the current writing type pixel structure is sensitive to noises, such as variation of a threshold voltage, and an irregular voltage rise at an earth line.
FIG. 1 illustrates a related art circuit of a voltage writing type pixel structure of two positive elements.
Referring to FIG. 1, there is a charge storage capacitor Cstg having both a drive transistor Q1 for direct driving of an organic electroluminescent (OEL) and a positive power source VDD connected thereto, for storage of charge of a TFT-LCD (Thin Film Transistor-Liquid Crystal Display). The drive transistor Q1 has one side connected to an anode of the OEL. There is a switching transistor Q2 having a gate connected to a scanline for switching the OEL under the control of a signal from the scanline. The switching transistor Q2 has a source connected to a dataline, a drain connected to a gate of the drive transistor Q1. The charge storage capacitor Cstg is connected both to the positive power source VDD and the gate of the drive transistor Q1. As shown in FIG. 1, the drive transistor Q1 and the switching transistor Q2 are PMOS (P type Metal Oxide Semiconductor)
The operation of the circuit in FIG. 1 will be explained.
A data voltage having a gray scale adjusted is provided from the dataline both to the charge storage capacitor Cstg and the gate of the drive transistor Q1 through the switching transistor Q2. When the switching transistor Q2 is closed in response to the scanline signal, a data voltage of the gray scale of each pixel is written on the charge storage capacitor Cstg through the dataline. The written data voltage is used as a control voltage for fixing a current level of the drive transistor Q1. The current by the control voltage is provided to the OEL through the drive transistor Q1. The AMOEL panel has lots of pixels, wherein, if voltage-current characteristics of the drive transistors Q1 between the pixels are not uniform, currents to the OELs in the pixels are not uniform, even if the voltages written on the charge storage capacitors Cstg are the same, which results in a non-uniform display, i.e., non-uniform luminance, on the AMOEL display panel, that is one of disadvantages of the voltage writing type.
FIG. 2 illustrates a circuit of a related art pixel of the current writing type. Different from the voltage writing type shown in FIG. 1, a current level of the gray scale is written on the drive transistor P1, directly.
Referring to FIG. 2, if a data drive circuit for providing a write current Idata is operable uniformly, the organic EL panel can display uniformly, even if the voltage-current characteristics of the drive transistors P1 of the pixels are not uniform. However, FIG. 2 illustrates a data drive circuit for only one pixel, actually. That is, a part for providing the writing current is present, not as only one circuit in the data drive circuit part, but for every dataline, or a few datalines. Therefore, if there are errors among the circuits that provide the writing currents, the pixels of the current writing type can not be make the best use of their advantages, such that the organic EL panel fails to have uniform display characteristics.
For solving the problem of FIG. 2, a circuit illustrated in FIG. 3 may be used. FIG. 3 illustrates a circuit for mirroring a reference current source IEF for providing desired current sources. In this case, one reference current source is employed in the data drive circuit. However, referring to FIG. 3, if one reference current source is mirrored to all the datalines, the reference current source can not exactly be mirrored, if a distance between transistors that act as mirrors is too far from the reference current source.
Referring to FIG. 4, as another method, a circuit for correcting the reference current source IREF can be employed. In a case of this circuit, current source devices, such as transistors, and charge storage capacitors may be used for making calibration periods equal for the datalines. However, a current leakage between a gate and a source of the charge storage transistor causes voltage variations on the datalines, and non-uniform output currents between the datalines.